A dynamic focus circuit is employed in CRT display monitors to make the focal length longer at the periphery of CRT screens by adjusting focusing of the electron beam from the electron lens of the electron gun in the CRTs, thereby improving focusing at all areas on CRT screens from the center to periphery. The dynamic focus circuit of the prior art achieves such adjustment by superimposing parabolic voltage modulated by the frequency of horizontal deflection and vertical deflection on the focus voltage to be applied to the focus electrode in the electron gun.
FIG. 3 shows an electrical block diagram of an example of the dynamic focus circuit of the prior art. In the dynamic focus circuit of the prior art, as shown in FIG. 3, the output of a vertical parabolic focus voltage generator 31, which has a higher voltage at the upper and lower ends in vertical deflection, and the output of a horizontal parabolic focus voltage generator 32, which has a higher voltage at the right and left ends in horizontal deflection, are sent to a multiplier 33. The output of the multiplier 33 and the output of the vertical parabolic focus voltage generator 31 are then added and amplified in an addition and amplification unit 34. The added and amplified output is finally applied to a focus electrode 36 in an electron gun of a CRT 35. With the above configuration, the dynamic focus circuit focus circuit of the prior art achieves an optimum focus over the entire screen even for CRT screens with large radius of the curvature. See, for example, Japanese Laid-open Patent No. H5-199428.
Requirements have recently arisen, however, for waveforms other than a parabolic waveform for use in correcting the focus voltage to an optimum form, such as in a case of a completely flat screen where the radius of the curvature of a CRT is considered to be infinite.
In addition, a high frequency has recently been employed as the horizontal deflection frequency for reducing flickering of CRTs. This requires a more complicated and expensive circuit configuration for the analog multiplier to produce the focus voltage waveform due to technical reasons including incidental oscillation. Furthermore, the configuration of the analog amplifier may require a variable resistor or a variable amplifier, resulting in the configuration requiring expensive bipolar or Bi-CMOS discrete components. In other words, the configuration of the analog multiplier in the prior art may not be realizable by employing a monolithic IC, resulting in difficulty in reducing the surface mounting area of a printed circuit board. The conventional configuration may not also be realizable with an IC using an inexpensive C-MOS process.